Voters in Mississippi will decide on Tuesday whether to retire a veteran U.S. senator who has steered billions of government dollars to his impoverished state, in the latest showdown between the Republican establishment and its insurgent Tea Party wing.

The election, a runoff between Sen. Thad Cochran and challenger state Sen. Chris McDaniel, has become a multimillion-dollar referendum on the direction of the Republican Party. This year’s primary season has pitted the party’s business-friendly wing against groups that place a premium on small government.

Senior Republican lawmakers in Kentucky, Idaho, and Texas, aided by the U.S. Chamber of Commerce and other business groups, have turned back primary challengers who argued that the incumbents are too willing to compromise with President Barack Obama and his fellow Democrats.

But the stunning upset two weeks ago of Representative Eric Cantor, the No. 2 Republican in the House of Representatives, by a little-known candidate showed that clout in Washington is no guarantee of victory back home.

Cochran, 78, has been re-elected by wide margins since he first won a seat in Congress in 1972, and courthouses and research centers throughout the state bear his name.

Cochran has channeled billions of federal dollars to Mississippi for shipbuilding, highways, crop subsidies, disaster relief, and other projects. The state, which has the lowest median income in the United States, depends on that money, and the jobs it creates, for nearly half of its budget.

McDaniel argues that Cochran’s nuts-and-bolts approach is out of step with voters in his deeply conservative state.

“We’re not going to tear down the government. We’re going to make it back to its constitutional limitations,” he said at a campaign stop in Meridian.

Cochran has spent $4 million so far on his re-election, and business groups such as the National Association of Realtors have poured in another $4 million.

McDaniel has spent $1.5 million, but he has been helped by more than $7 million in outside spending by conservative groups such as the Club for Growth.

McDaniel edged Cochran in a June 4 primary but failed to win 50 percent of the vote, prompting the runoff.

The winner will be strongly favored to defeat Democrat Travis Childers in the Nov. 4 election, although Cochran’s supporters have argued that sexually suggestive remarks made by McDaniel on a radio show could make the race competitive.

Republicans need to pick up six seats to win control of the 100-seat U.S. Senate, which would give them greater leverage to oppose Obama’s agenda during his remaining two years in the White House. They are expected to retain control of the House of Representatives.